1. Field of the Invention
This invention relates to a double-tuned circuit and, more particularly, to improvements of the bandwidth and the flatness of the passband characteristic in high receive channels of the highband.
2. Description of the Prior Art
In television tuners, double-tuned circuits having a bandwidth necessary for a television signal and a good selectivity are used to select receive channels and suppress unnecessary disturbance waves.
FIG. 5 is a circuit diagram of an example of conventional double-tuned circuits. In FIG. 5, an input end 2 of a double-tuned circuit 1 is grounded through a dc-blocking capacitor 3 and a variable-capacitance diode 4 in series and through first, second and fifth resonance coils 5, 6 and 7 and a dc-blocking capacitor 8 also in series. The connection point of the first and second resonance coils 5 and 6 is grounded through a first switching diode 9 and a dc-blocking capacitor 10 in series. An output end 11 of the double-tuned circuit 1 is grounded through a dc-blocking capacitor 12 and a variable-capacitance diode 13 in series and is connected to the connection point of the second and fifth resonance coils 6 and 7 through third and fourth resonance coils 14 and 15 in series. The connection point of the third and fourth resonance coils 14 and 15 is grounded through a second switching diode 16 and a dc-blocking capacitor 17 in series. Further, the connection points of the first and second switching diodes 9 and 16 and the dc-blocking capacitors 10 and 17 are connected together through, for example, a printed pattern and to a highband switching voltage terminal 19 through a resistor 18. Furthermore, the connection points of the dc-blocking capacitors 3 and 12 and the variable-capacitance diodes 4 and 13 are connected to a tuning voltage terminal 22 through resistors 20 and 21, respectively. In addition, the connection point of the fifth resonance coil 7 and the dc-blocking capacitor 8 is connected to a lowband switching voltage terminal 24 through a resistor 23. Of course, the first and third resonance coils 5 and 14, as well as the second and fourth resonance coils 6 and 15, are disposed so as to effect inductive-coupling therebetween.
In the foregoing configuration, as a positive voltage is applied to the lowband switching voltage terminal 24 and the highband switching voltage terminal 19 is grounded, both the first and second switching diodes 9 and 16 are reverse-biased to be turned off. As a result, a primary tuning circuit of the lowband is formed by the first, second and fifth resonance coils 5, 6 and 7, variable-capacitance diode 4, and dc-blocking capacitors 3 and 8, whereas a secondary tuning circuit of the lowband is formed by the third, fourth and fifth resonance coils 14, 15 and 7, variable-capacitance diode 13, and dc-blocking capacitors 8 and 12. On the contrary, as a positive voltage is applied to the highband switching voltage terminal 19 and the lowband switching voltage terminal 24 is grounded, both the first and second switching diodes 9 and 16 are forward-biased to be turned on. As a result, a primary tuning circuit of the high-band is formed by the first resonance coil 5, variable-capacitance diode 4, and dc-blocking capacitors 3 and 10, whereas a secondary tuning circuit of the highband is formed by the third resonance coil 14, variable-capacitance diode 13, and dc-blocking capacitors 12 and 17. Accordingly, by regulating a tuning voltage being applied to the tuning voltage terminal 22, the primary and secondary tuning circuits of the lowband/highband can be adjusted to a receive channel.
In the foregoing double-tuned circuit 1 shown in FIG. 5, in order to make high the Q (quality factor) of the primary and secondary tuning circuits at the time of reception of the highband, the connection point of the variable-capacitance diodes 4 and 13 is disposed at the same point as that of the connection point of the dc-blocking capacitors 10 and 17 to shorten the connection path. However, owing to a limitation in layout of parts th first and second switching diodes 9 and 16 are disposed in spaced relation to each other, thus, a printed pattern for connecting together the anodes of these diodes becomes long.
Consequently, at the time of reception of the high-band, as shown in the equivalent circuit diagram of FIG. 6, between the first and third resonance coils 5 and 14 is interposed an inductance component 25 of the printed pattern for connection of them. Further, a stray capacity 26 pertinent to a print circuit board etc. is created between the first and third resonance coils 5 and 14.
The bandwidth of the double-tuned circuit 1 varies depending upon the degree of coupling between the primary and secondary tuning circuits, and this degree of coupling increases due to the inductance component 25 of the printed pattern with an increase of the receive channel frequency. Therefore, as shown in FIG. 7, the bandwidth, for example, of 6 MHz at the receive channel being 100 MHz, changes to 15 MHz at 200 MHz, or to 25 MHz at 300 MHz. In this way, the bandwidth enlarges unnecessarily to thereby pass unwanted signals falling within an enlarged bandwidth to a succeeding stage. Further, the Q of the primary and secondary tuning circuits increases with an increase of the receive channel frequency to thereby degrade the flatness of the passband characteristic. Accordingly, the conventional double-tuned circuit 1 has the problem that the performance of reception is degraded with an increase of the receive channel frequency.